How does an Isotope Stress
Test Work? An isotope stress test is also
known as a nuclear, thallium, Cardiololite, Myoview
or dual isotope stress test, depending upon the method
used.
During exercise, healthy
coronary arteries dilate (develop a more open channel)
more than an artery that has a blockage. This unequal
dilation causes more blood to be delivered to heart
muscle supplied by the normal artery. In contrast,
narrowed arteries end up supplying reduced flow to
its area of distribution. This reduced flow causes
the involved muscle to "starve" during exercise.
The "starvation" may produce symptoms (like
chest discomfort or inappropriate shortness of breath),
and EKG abnormalities. When a "perfusion tracer"
(a nuclear isotope that travels to heart muscle with
blood flow) is injected intravenously, it is extracted
by the heart muscle in proportion to the flow of blood.
The amount of tracer uptake helps differentiate normal
muscle (which receives more of the tracer) from the
reduced uptake demonstrated by muscle that is supplied
by a narrowed coronary artery. In other words, areas
of the heart that have adequate blood flow quickly
picks up the tracer material. In contrast, muscle
with reduced blood flow pick up the tracer slowly
or not at all. Analysis of the images of the heart
(taken by a scanning camera) can help identify the
location, severity and extent of reduced blood flow
to the heart. The reduced blood flow is known as ischemia
(pronounced is-keem-ya).

How
is an Isotope or Nuclear Stress Test performed?
The test is actually divided into three parts: A
treadmill stress test, imaging at rest, and imaging
after exercise. There are two common types of isotope
used in the USA. They are thallium and technetium
(which are marketed under the Trade names "Cardiolite"
and "Myoview"). Some laboratories use
a "dual isotope" technique, where thallium
is used for the resting images and technetium is
used for the stress pictures. Depending upon the
isotope and protocol for the laboratory, resting
images may be obtained either before stress or two
to four hours after stress. The preparation for
the test and the treadmill procedure is similar
to that described under the Regular Treadmill Stress
Test section. In patients who are unable to complete
a high level of exercise because of physical limitations,
stress to the heart is provided by pharmaceutical
or chemical stimulation.

The patient is brought
to the exercise laboratory where the heart rate and
blood pressure are recorded at rest. Sticky electrodes
are attached to the chest, shoulders and hips and
connected to the EKG portion of the Stress test machine.
A 12-lead EKG is recorded on paper. Each lead of the
EKG represents a different portion of the heart, with
adjacent leads representing a single wall (see Regular
Treadmill for details). The treadmill is then started
at a relatively slow "warm-up" speed. The
treadmill speed and it's slope or inclination are
increased every three minutes according to a preprogrammed
protocol. Bruce is the commonest protocol in the USA,
but several other protocols are perfectly acceptable
(see Regular Treadmill for details). . It is the protocol
that dictates the precise speed and slope. Each three
minute interval is known as a Stage (Stage 1, Stage
2, Stage 3, etc. Thus a patient completing Stage 3
has exercised for 3 x 3 = 9 minutes). The patient's
blood pressure is recorded during the second minute
of each Stage. However, it may be recorded more frequently
if the patient's reading is outside too high or too
low.

The EKG is constantly displayed
on the monitor. It is also recorded on paper at one
minute intervals. The physician pays particular attention
to the heart rate, blood pressure, changes in the
EKG pattern, irregular heart rhythm, and the patient's
appearance and symptoms. The treadmill is stopped
when the patient achieves a target heart rate (this
is 85% of the maximal heart rate predicted for the
patient's age). However, if the patient is doing extremely
well at peak exercise, the treadmill test may be continued
further. The test may be stopped prior to achievement
of the target heart rate if the patient develops significant
chest discomfort, shortness of breath, dizziness,
unsteady gait, etc., or if the EKG shows alarming
changes or serious abnormal heart rhythm. It may also
be stopped if the blood pressure (BP) rises or falls
beyond acceptable limits.

Approximately one to 1 1/2
minutes prior to termination of exercise, the perfusion
tracer or isotope is injected into the intravenous
"plug" that had been placed in the forearm
or hand. This is followed by a "flush" injection
of saline (salt water) to make sure that all of the
tracer is pushed into the blood circulation. After
a brief waiting phase (that allows the tracer to be
taken up by the heart muscle) the patient is placed
under a scanning camera.

Two sets of isotope images are obtained.
One at rest, and one following exercise. Depending
upon the isotope used and the protocol for a particular
laboratory, the resting images may be obtained before
the stress test, or a few hours later. The scanning
camera rotates around the patient's chest, stopping
to take individual pictures. The patient needs to
lay flat and still during the scanning period which
takes approximately 11 to 20 minutes, depending upon
the type of scanning camera. Patients with severe
claustrophobia should notify their physician (a mild
tranquilizer before the test may minimize discomfort).
The pictures or images are fed into a computer, which
reconstructs them as "slices" of a three
dimensional heart. These slices are presented in three
views (vertical long axis or VLA, horizontal long
axis or HLA and short axis or SA). It also computes
the data and presents an aggregate "bull's eye"
picture that compares the information to a data base
of known normal cases. Areas that fall out side the
expected normal range is presented as a blacked out
area. In other words, your physician has an opportunity
to view a three dimensional representation of your
heart, examine individual "slices" and then
compare the findings against those computed by the
computer as "blackout plots. By comparing one
wall against another, the physician can identify disease
and assess its magnitude.

Let us pause and examine
actual thallium stress test image(above). First, let
us look at the three standard views (VLA, HLA and
SA, as discussed above) of the left ventricle. In
the VLA view one sees the anterior wall on top and
the inferior wall is below. The two walls meet at
the apex or tip of the heart. In the HSA view, the
septum is to the left and the lateral wall is to the
right and they meet at the apex. In the SA view, the
anterior, inferior, septum and lateral walls are above,
below, left and right, respectively (see above).

The "pizza" appearing picture on the far
right is the "bull's eye" image. The center
of the bulls eye is the apex of the heart. The computer
"slices" the heart in the SA view and "lays"
the slices or rings down, one by one, beginning at
the tip and moving upwards to the base (widest portion)
of the heart. The representative walls are similar
to that seen in the SA view. Colors ranging from orange
to white are "hot colors" and represent
normal blood flow. Dark shades of orange, brown, blue
and black are "cold" colors and represent
little to no flow. Ignore the bluish halo surrounding
the heart slices above because they are NOT WITHIN
the walls of the left ventricle (LV).

The physician can separate a normal left ventricle,
from ischemia (live muscle with flow that is compromised
only during exercise) and the scar tissue of a heart
attack. The distinction is made in the following way:

NORMAL = Both the resting and stress images show
uniform uptake through out the heart, as shown above.
In comparison, the rest images do not show a localized
area of increased isotope uptake. These findings are
demonstrated in the stress (top) and rest (bottom)
slices or tomograms (tomos; the round pizza shaped
images).
The top two circular images
on the left show normal uniform uptake of the isotope.
The entire heart is either a bright shade of orange,
yellow or white. The blue outer halo is outside the
walls of the LV and have no clinical importance.
Since everything is within normal limits, the lower
images do not demonstrate any blacked out area.

ISCHEMIA: = The top strip of the
tomos or tomograms (stress images) are abnormal. They show dark
colors in the inferior wall (bottom surface) and apex or tip (VLA view),
and lateral wall in the HSA view and the lower lateral
wall in the SA view. Thus, there is evidence of reduced
blood flow to the inferior, apex and lateral walls.
In comparison, the rest images show near normalization
of all three segments (which regain brighter colors).
This is a typical finding in a patient with ischemia
or reversible stress-induced reduction of blood supply
to the LV. The four circular or bulls-eye images confirm
these findings. The top left circle shows reduced
flow (darker colors) in the apex, inferior and lower
(posterior) lateral wall of the LV. These areas brighten
up (normalize) in the top right (Rest) view. This
patient has not had a heart attack but the walls are
threatened or jeopardized. The blackened areas in
the lower two circles are generated by the computer,
which conveniently marks the abnormal areas in the
Stress (lower left) but not in the "normalized"
Rest (lower right) image.

HEART ATTACK: = The top strip of the tomo slices
(stress images) show that the inferior, apex and lower
lateral walls of the LV have dark colors. It also
appears thinner than the normal walls because of reduced
blood flow and consequent decrease in tracer uptake.
Notice that these findings persist in the rest or
lower strips. In other words, there is no reversibility
of flow in the Rest images. Such findings indicate
a previous heart attack. Also, in the absence of reversibility
there is no evidence of additional threatened or jeopardized
heart muscle. The four circular or bulls-eye images
confirm these findings. The top left circle shows
reduced flow (darker colors) in the apex, inferior
and lower (posterior) lateral wall of the LV. The
changes are fixed and do not change in the rest images
(top right). The blackened areas in the lower two
circles are generated by the computer which conveniently
marks the abnormal areas. Once again, there is no
significant change between the Stress and Rest pictures
(unlike what we observed in the case of ischemia).

Preparing
for an Isotope Stress Test: The following
recommendations are "generic" for all types
of cardiac stress tests:

Do not eat or drink for three hours prior to the
procedure. This reduces the likelihood of nausea
that may accompany strenuous exercise after a heavy
meal. Diabetics, particularly those who use insulin,
will need special instructions from the physician's
office.

Specific heart medicines may need to be stopped
one or two days prior to the test. Such instructions
are generally provided when the test is scheduled
by the doctor's office. Call if you have any questions.

Wear comfortable clothing and shoes that are suitable
for exercise.

An explanation of the test is provided and the
patient is asked to sign a consent form.

How long does the entire
test take? A patient should allow approximately
two to four hours for the entire test, including the
preparation. Dual isotope and technetium stress testing
takes less time than thallium. You will be allowed
to leave the lab and get a light snack or lunch in
cases of thallium stress testing.

How safe is an Isotope
Treadmill Stress Test? The patient is exposed
to a very small amount of radiation and the risk is
minimal, if any. The risk of the stress portion of
the test is very small and similar to what you would
expect from any strenuous form of exercise (jogging
in your neighborhood, running up a flight of stairs,
etc.). As noted earlier, experienced medical staff
is in attendance to manage rare complications like
sustained abnormal heart rhythm, unrelieved chest
pain or even a heart attack. In such cases, the patient
is better off having the problem in the presence of
experienced staff, rather than have it happen when
they are exercising alone.

What is the reliability
of an Isotope Stress Test? If a patient is
able to achieve the target heart rate and good quality
images are obtained, an isotope treadmill stress test
is capable of diagnosing important disease in approximately
85% of patients with coronary artery Approximately
10% of patients may have a "false-positive"
test (when the result is falsely abnormal in a patient
without coronary artery disease). Technical problems
can occur when a patient is markedly overweight. Women
may have an abnormality in the front portion of the
heart because of overlying breast tissue. Some men
may demonstrate an inferior wall abnormality because
of a prominent diaphragm (muscular partition that
separates the chest cavity from the abdomen). Patients
who have a left bundle branch block on their EKG may
also have a false abnormal test. How quickly will I get the
results and what will it mean? The physician
performing the stress test can give you a preliminary
report about the EKG portion of your test. However,
the official result from the isotope scans may take
a few days to complete. The results may influence
your physician's decision to change your treatment
or recommend additional testing such as cardiac catheterization,
or a change in your medications.

Panoramic View of
Patient Having an Isotope Stress Test

You may pause, pan left and right by dragging your
mouse within the panoramic picture.